Nowadays, X-ray imaging is subject to stringent requirements for the image quality of the recordings. In these recordings, such as those which are made in particular for medical X-ray diagnosis purposes, an object to be examined has X-ray radiation passed through it from an X-ray source which is approximately in the form of a point. The attenuation distribution of the X-ray radiation is recorded two-dimensionally on that face of the object which is opposite the X-ray source. In addition, in the case of computer tomography, the X-ray radiation that has been attenuated by the object is recorded line-by-line.
Solid-state detectors are increasingly being used as radiation detectors, in which semiconductor elements which act as receivers are arranged in the form of a matrix. The X-ray recording which is obtained is composed of a large number of individual pixels, with each pixel ideally corresponding to the attenuation of the radiation through the object on a linear axis from the X-ray source to that location on the detector surface which corresponds to the respective pixel. The radiation which strikes the detector on this linear axis is referred to as primary radiation.
However, as the X-ray beams pass through the object, they necessarily interreact with the object, leading to scatter effects. Thus, in addition to the actual primary beams which pass through the object without being scattered, secondary beams occur which strike the detector from directions other than the linear axis. These secondary beams, which can make up a very large portion of the total signal drive level of the detector, represent an additional noise source and reduce the capability to identify fine contrast differences.
It is known for so-called antiscatter grids to be used in order to reduce the scattered radiation striking the detector. Known antiscatter grids include regularly arranged structures which absorb X-ray radiation and between which aperture channels or the like are provided for primary radiation. In this case, a distinction is drawn between focused grids and unfocused grids. In the case of focused grids, the aperture channels and hence the absorption structures which bound them, are aligned with the focus of the X-ray source. However, this is not the case with unfocused grids, in which the channels are at right angles to the surface.
An antiscatter grid operates in such a way that the secondary radiation is primarily absorbed via the absorbent structures. Further, in the case of unfocused grids, a proportion of the primary radiation is also absorbed by them. Thus, it is not part of the proportion of the radiation that strikes the detector and produces the actual X-ray image. The aim in this case is always on the one hand for the scattered beams to be absorbed as well as possible while, on the other hand, having as high a proportion of the primary radiation as possible passing through the grid without being attenuated.
A reduction in the scattered beam components can be achieved by use of a high aspect ratio; namely a high ratio of the height of the grid to the thickness or to the diameter of the aperture channels. However, the thickness of the absorbent elements which are located between the channels makes it possible, in particular, for image interference to be caused by absorption of a portion of the primary radiation. Particularly when using the grid in conjunction with a matrix detector, any inhomogeneity in the grid leads to image interference as a result of the grid being imaged in the X-ray image. There is a risk here of the projection of the structures of the detector elements and of the antiscatter grid interfering with one another, as a result of which Moiré interference phenomena can occur.
These problems also occur in the case of a grid such as that described in the previously published patent application DE 102 41 424.6. Here, a novel grid type is described, in comparison to the conventional lead laminate grids. Conventional lead laminate grids are so-called “laid grids”, in which very thin lead laminates and elements which are generally composed of paper, effectively form the aperture slots between the laminates and are transparent to radiation are laid alternately. However, the manufacturing precision for grids such as these is limited, leading to problems in particular in the case of solid-state detectors.
This is in contrast to the grid from DE 102 41 426, which is produced by use of a rapid prototyping technique by solidification of an applied material in layers. This technique allows very fine and exact structures to be formed, which are the basis for the formation of the absorption structure. These structures are formed corresponding to the profile of the insensitive intermediate areas between two detector elements in the solid-state matrix detector. Namely, these structures to which an absorption coating is applied run exactly over these intermediate areas and not above the active detector surface. Although the capability to identify raster images can be reduced with this known grid and can be shifted to such a high spatial frequency range that it is still barely possible to image them sharply by use of the imaging systems, the geometry of the absorption structure nevertheless results in difficulties which can lead to the formation of Moiré phenomena in the radiation image which is produced.
Similar difficulties to those in X-ray diagnostics also occur in nuclear medicine, particularly when using gamma cameras. In this case as well, care must be taken to ensure that as few scattered gamma quanta as possible reach the detector. In this type of examination, the radiation source for the gamma quanta is located in the interior of the object being examined. After injection of an unstable nuclide, an image of an organ is produced by detection of the quanta which are emitted from the body by the nuclide decomposition, with the time profile of the activity or the decomposition in the organ allowing conclusions to be drawn about its operation.
In this technique, a collimator which defines the projection direction of the image is placed in front of the gamma detector, corresponding to an antiscatter grid. The method of operation and structure of this collimator essentially correspond to those of the antiscatter grid described initially.